From: Eric Dumazet <edumazet@google.com>
Date: Fri, 29 May 2020 11:32:25 -0700

syzbot was able to trigger a crash after using an ISDN socket
and fool l2tp.

Fix this by making sure the UDP socket is of the proper family.

 ...

Fixes: 6b9f34239b00 ("l2tp: fix races in tunnel creation")
Fixes: fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: James Chapman <jchapman@katalix.com>
Cc: Guillaume Nault <redacted>
Reported-by: syzbot <redacted>

Applied and queued up for -stable, thanks.